Hi, Reading the ceph-users list I'm obviously seeing a lot of people talking about using bluestore now that Luminous has been released. I note that many users seem to be under the impression that they need separate block devices for the bluestore data block, the DB, and the WAL... even when they are going to put the DB and the WAL on the same device!
As per the docs at http://docs.ceph.com/docs/master/rados/configuration/bluestore-config-ref/ this is nonsense: > If there is only a small amount of fast storage available (e.g., less than a > gigabyte), we recommend using it as a WAL device. If there is more, > provisioning a DB > device makes more sense. The BlueStore journal will always be placed on the > fastest device available, so using a DB device will provide the same benefit > that the WAL > device would while also allowing additional metadata to be stored there (if > it will fix). [sic, I assume that should be "fit"] I understand that if you've got three speeds of storage available, there may be some sense to dividing these. For instance, if you've got lots of HDD, a bit of SSD, and a tiny NVMe available in the same host, data on HDD, DB on SSD and WAL on NVMe may be a sensible division of data. That's not the case for most of the examples I'm reading; they're talking about putting DB and WAL on the same block device, but in different partitions. There's even one example of someone suggesting to try partitioning a single SSD to put data/DB/WAL all in separate partitions! Are the docs wrong and/or I am missing something about optimal bluestore setup, or do people simply have the wrong end of the stick? I ask because I'm just going through switching all my OSDs over to Bluestore now and I've just been reusing the partitions I set up for journals on my SSDs as DB devices for Bluestore HDDs without specifying anything to do with the WAL, and I'd like to know sooner rather than later if I'm making some sort of horrible mistake. Rich -- Richard Hesketh
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